During the past decade, the propagation of powerful laser pulses over long distances in air has attracted a lot of interest. In particular, the optical processes occurring during the propagation of these pulses in the atmosphere are of great interest. For instance, the propagation behavior of powerful and ultrashort laser pulses in air differs significantly from that of continuous wave or long pulse lasers. For ultrashort and intense laser pulses, numerous nonlinear optical effects are produced such as the Kerr self-focusing effect, the self-generation of plasma, and the spectral broadening effect due to self-phase modulation. Other novel laser properties may be found from ultrashort and intense laser pulses, such as self-collimated beams over long distance, spatial beam self-cleaning, and self-stabilization.
More than being of high scientific interest, the filamentation of ultrashort and intense laser pulse has a number of important applications. The propagation of these intense ultrashort laser pulses in the atmosphere can induce the ionization of air and create a conductive plasma channel along its path. Such a plasma channel can be used to guide other energy sources, such as microwave and high voltage energy sources. The plasma channel can also be used for counter-measures against improvised explosives devices, rockets and other similar threats. The use of laser pulse filamentation enables the scanning of a large area or the quick aim of the laser in the direction of a threat. Moreover, the filamentation of a laser pulse modifies its spectral distribution during the propagation and evolves into a continuous broadband (white-light) laser pulse. The broadband pulse can be used for broadband dazzling of optical devices—independently of the spectral range used by the optical detector of the threat.
The propagation of a high power laser beam into the atmosphere is characterized by numerous linear and nonlinear optical effects that influence the transportation of the laser power at long distances. A nonlinear optical effect, such as the Kerr self-focusing, or a linear optical effect, such as the atmospheric turbulence, can strongly decrease the achievable laser intensity at long distance. As such, reaching a high laser intensity at long range in the atmosphere with a compact optical device remains a challenge. Solutions for delivering the high laser intensity at long distances involve the use of optical components that correct the perturbations from the linear propagation effects and that modify the parameters of the laser beam to minimize the nonlinear intensity-dependent optical effects.
Many approaches exist to control the filamentation distance of ultrashort and intense laser pulse.
The use of a refractive telescope to expand the laser beam diameter is disclosed in the following reference: W. Liu, F. Theberge, J.-F. Daigle, P. T. Simard, S. M. Sarifi, Y. Kamali, H. L. Xu, S. L. Chin, “An efficient control of ultrashort laser filament location in air for the purpose of remote sensing,” Applied Physics B, 85, 55 (2006). Here, a refractive beam expander is designed to control the filamentation distance of ultrashort and intense laser pulses. The generation of ultrashort and intense laser pulses at relatively long distances is done by increasing the radius of the beam and decreasing its peak power in such a way that the self-focusing distance zf is much longer than the effective focal length of the telescope feff.
Unfortunately, the refractive telescope method is only optimized for a single wavelength. Therefore, the pulse is subject to chromatic aberrations since the spectrum of ultrashort pulse is large (up to 10's of nm). Moreover, non-linear optical effects are more problematic in glass than in air, and the self-focusing within the refractive optics of the telescope tends to generate “hot spots” in the laser beam profile. These hot stops decrease the effective radius of the laser beam and cause the laser beam to self-focus at shorter distances. Another drawback with this prior art technique is that such refractive telescopes do not compensate for the aberrations from the laser beam itself and the perturbation from the atmospheric turbulences.
The following prior art reference discloses masking the edge of the laser beam with an iris: J.-F. Daigle, O. Kosareva, N. Panov, M. Bégin, F. Lessard, C. Marceau, Y. Kamali, G. Roy, V. P. Kandidov, S. L. Chin, “A simple method to significantly increase filaments' length and ionization density” Appl. Phys. B., 94, 249, (2009). The prior art reference teaches that by clipping the edge of a laser beam with the optimum aperture, it is possible to include multiple filaments around the propagation axis, thus forming a regularized and elongated filament structure with a higher overall amount of plasma.
One drawback of the clipping technique is that masking the edge of a laser beam with an iris may decrease the power of the laser beam. Unfortunately, the optimum iris diameter depends on the beam profile of the laser beam. Therefore, for real propagation in the atmosphere, the beam profile of the laser beam can strongly fluctuate due to the atmospheric turbulences, and it becomes impossible to determine an optimal iris diameter.
Another prior art reference discloses chirping the laser pulse to compensate for the dispersion of the atmosphere: M. Rodriguez, R. Bourayou, G. Méjean, J. Kasparian, J. Yu, E. Salmon, A. Sholz, B. Stecklum, J. Eislöffel, U. Laux, A. P. Hatzes, R. Sauerbrey, L. Wöste, J. P. Wolf, “Kilometer-range non-linear propagation of fs laser pulses,” Physical Review E 69, 036607 (2004). By chirping the laser pulse (i.e., increasing its pulse duration) from the initial laser source, it is possible to decrease the peak power P of the laser pulse and pre-compensate for the dispersion from the atmosphere in order to control the self-focusing distance, i.e., control the range of filamentation. However, the self-focusing distance zf is proportional to the peak power P and the square of the beam radius a. Therefore, increasing the size of the beam is the most efficient way to increase the self-focusing distance, rather than only reducing the peak power. Any aberrations from the laser beam itself or any perturbations from the atmospheric turbulence are not compensated for by this prior art technique.
As suggested, the techniques utilized in the prior art are unsuitable or lack suitable efficiency for controlling the laser beam filamentation at long distances in the atmosphere.